Diversity, Ecology and population dynamics of Lepidopteran stem borers in Kenya

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Stem borers are important field insect pests of maize [Zea mays L.] and sorghum [Sorghum bicolor (L.) Moench] in Africa. They account for more than 30% yield losses depending on fhe composition of the pest community. A total of 21 pest species have been reported in sub-Saharan Africa all of which are indigenous to the continent except for Chilo partellus (Swinhoe), which was accidentally introduced from Asia. Stem borers are susceptible to environmental fluctuations and the pest species are thought to have experienced changes in physiology and behaviour after close association with highly nutritive crops. Recent studies indicate that in addition to the pest species, there are noneconomic stem borer species among wild hosts in the uncultivated fragments. Owing to susceptibility of stem borer species, continued habitat fragmentation and degradation may ultimately result in host range expansion and eventual emergence of "new" pests. Unfortunately, previous studies have been geared towards reducing populations of pest species in the cultivated fields with few attempts to understand possible evolution of less known species to pest status. This research was therefore designed to gather information on stem borer species diversity, host range and ecology in selected agricultural landscapes in Kenya. Surveys were conducted during 2005/2007 growing seasons in and around selected cultivated fields in four localities, Muhaka, Mtito Andei, Kakamega and Suam, representing different altitudinal gradients across the country. A total of 29 stem borer species were identified from 9,771 larvae collected. The identified stem borer species were grouped into 10 different known genera (Acrapex, Busseola, Carelis, Manga, Poecopa, Sciomesa, Sesamia, Eldana, Chilo and Ematheudes) while the unknown species belonged to five different families (Crambidae, Peoriinae, Pyralidae, Schoenobiinae and Tortricidae). There was evidence of variation in both distribution and dominance among the surveyed localities with majority of the species belonging to the Noctuidae family found in Kakamega and Suam, while species belonging to Crambidae and Pyralidae were mainly found in Muhaka and Mtito Andei. The wild stem bore species were identified from 38 different plant species belonging to three different families (Cyperacea [27], Poaceae [10] and Typhaceae [1]), while pest species, Busseola fusca (Fuller), Sesamia calamistis Hampson, Chilo partellus Swinhoe, Chilo orichalcociliellus (Strand) and Eldana saccharina Walker were mainly found on maize and sorghum. Sesamia calamistis and B. phaia ssp. phaia occurred among both wild and cultivated hosts and provided good models for studying exchange of stem borer pest populations between the wild and cultivated habitats. Cytochrome b gene sequences, through the existence of strong genetic structuration, revealed evidence of limited exchange of S. calamistis populations between the habitats. However, genetic analyses of the same gene of Busseola phaia ssp. phaia Bowden populations revealed weak differentiation with respect to host use in different habitats (FsT = 0.016; P = 0.015). Observed variations in the distribution of pest and non-pest stem borer species coupled with differences in genetic structure among model species (S. calamistis and B. phaia ssp. phaia) suggest two things; i) no single management strategy would apply across different landscapes and ii) continued habitat fragmentation 1 degradation would affect ecosystem stability resulting in host range expansion or local species extinctions. Similar intensive studies need to be extended to other areas as it will form the basis upon which different integrated pest management (IPM) packages could be developed.